[show abstract][hide abstract] ABSTRACT: The structure and the magnetocaloric effect of single-phased Mn0.997Fe0.003As were studied with X-ray diffraction and vibrating sample magnetometry. In the Mn0.997Fe0.003As sample, the firstorder ferromagnetic-to-paramagnetic transition was observed near the Curie temperature (T
), namely at 308 K for the quenched sample and at 313 K for the slowly-cooled sample. This magnetic transition was accompanied by a structural transition from a hexagonal (NiAs-type) to an orthorhombic (MnP-type) structure. We also observed that after the heat treatment, the sample showed a large change in the magnetocaloric effect depending on the cooling conditions. From the isothermal M-H curves, the changes in the magnetic entropy (−ΔS
) were determined at temperatures between 280 and 320 K for different magnetic fields. For the sintered samples under slow cooling and water quenching, the maximum magnetic entropy changes at a magnetic field of 1.5 T were 19.6 and 32.2 J/kg K, respectively. Such a significant difference between the maximum entropy changes is due to the degree of the structure distortion, which depends on the heat treatment.
Journal- Korean Physical Society 08/2013; 60(7). · 0.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: The crystallographic and the magnetocaloric effects of La0.7Ca0.3Mn0.9957Fe0.01O3 (LCMO) powders were studied by using X-ray diffraction, vibrating sample magnetometer, and Mössbauer spectroscopy. To investigate the effect of sintering conditions on LCMO, the samples were sintered in air and in an evacuated sealed quartz tube at different temperatures. The calculated magnetic entropy changes in a magnetic field of 1.5 T for the samples sintered in air and in an evacuated sealed quartz tube were 1.6 J/kg K and 3.2 J/kg K, respectively. The magnetic entropy change increased considerably with increasing annealing temperature, but the Curie temperature T
slightly decreased. However, the vacuum-annealed sample showed increases in both the T
and the magnetic entropy change. The magnetocaloric effect and the working temperature of lanthanum manganites are tunable by using annealing conditions.
Journal- Korean Physical Society 08/2013; 61(11). · 0.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: The olivine-structured Fe1−y
PO4 (y = 0.0, 0.1, and 0.3), a possible cathode material for lithium-ion secondary battery, has been studied by using x-ray diffraction (XRD) and Mössbauer spectroscopy. These Fe1−y
PO4 samples were prepared by using the chemical lithium deintercalation process from LiFe1−y
PO4. The crystal structures of the Fe1−y
PO4 samples were determined to be orthorhombic (space group Pnma) at room temperature by using the Rietveld refinement method. From the Mössbauer spectra at room temperature, the electric quadrupole splitting (ΔE
) and the isomer shift (δ) values of the Fe1−y
PO4 were determined to be ΔE
= 1.512 mm/s, and δ = 0.31 mm/s for y = 0.0, ΔE
= 1.502 mm/s, and δ = 0.31 mm/s for y = 0.1, and ΔE
= 1.382 mm/s, and δ = 0.31 mm/s for y = 0.3. The difference in the ΔE
values for Fe1−y
PO4 samples with varying Mn concentrations can be explained by the change in the exchange interaction due to the dependence of the asymmetry in the FeO6 octahedral sites on the Mn concentration.
Journal- Korean Physical Society 08/2013; 62(12). · 0.51 Impact Factor
[show abstract][hide abstract] ABSTRACT: A methodology providing access to dumbbell-tipped, metal-semiconductor and metal oxide-semiconductor heterostructured nanorods has been developed. The synthesis and characterization of CdSe@CdS nanorods incorporating ferromagnetic cobalt nanoinclusions at both nanorod termini (i.e., dumbbell morphology) are presented. The key step in the synthesis of these heterostructured nanorods was the decoration of CdSe@CdS nanorods with platinum nanoparticle tips, which promoted the deposition of metallic CoNPs onto Pt-tipped CdSe@CdS nanorods. Cobalt nanoparticle tips were then selectively oxidized to afford CdSe@CdS nanorods with cobalt oxide domains at both termini. In the case of longer cobalt-tipped nanorods, heterostructured nanorods were observed to self-organize into complex dipolar assemblies, which formed as a consequence of magnetic associations of terminal CoNP tips. Colloidal polymerization of these cobalt-tipped nanorods afforded fused nanorod assemblies from the oxidation of cobalt nanoparticle tips at the ends of nanorods via the nanoscale Kirkendall effect. Wurtzite CdS nanorods survived both the deposition of metallic CoNP tips and conversion into cobalt oxide phases, as confirmed by both XRD and HRTEM analysis. A series of CdSe@CdS nanorods of four different lengths ranging from 40 to 174 nm and comparable diameters (6-7 nm) were prepared and modified with both cobalt and cobalt oxide tips. The total synthesis of these heterostructured nanorods required five steps from commercially available reagents. Key synthetic considerations are discussed, with particular emphasis on reporting isolated yields of all intermediates and products from scale up of intermediate precursors.
[show abstract][hide abstract] ABSTRACT: Monodispersed Fe3−xMnxO4 (x = 0, 0.25, 0.5, 0.75, and 1.0) microspheres were prepared by a solvothermal reaction method. Field emission scanning electronmicroscope (FESEM) and high resolution transmission electron microscope (HRTEM) measurements showed that the size of the monodispersed particles was around 200 ∼ 400 nm, with single crystalline spots in the selected-area electron diffraction (SAED) patterns. From the detailed Rietveld refinement analysis, the crystal structure was determined to be cubic spinel with lattice constant a0, linearly increasing from 8.3956 to 8.4319 Å with the Mn concentration. Also, with Mn concentration, saturation magnetization decreased from 76.9 to 60.3 emu/g at 295 K and 99.5 to 78.4 emu/g at 4.2 K, while coercivity decreased from 66 to 36 Oe at 295 K and 271 to 185 Oe at 4.2 K. We have analyzed the recorded Mössbauer spectra as 3 sets with six lines of tetrahedral A site and octahedral B1 and B2 sites at 295 K and 4.2 K. From the isomer shift values, the valence state of the A and B1 site was determined to be ferric, while the B2 site was ferrous. The corresponding area ratio of the A site increased from 40 to 50%, while that of the B site decreased from 60 to 50% as the Mn concentration changed from x = 0 to 1.0. Here, the changes in the area ratios of A and B sites are due to the changes in the cation distributions at the A and B sites, originating from the randomly substituted Mn ions in Fe3−xMnxO4 microspheres.
Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Recently, lithium iron sulfate compounds have been highly considered for a positive electrode material for lithium ion batteries. In this work, LiFeSO4F and the fully delithiated FeSO4F were prepared by the ionothermal process and chemical oxidation reaction method. The LixFeSO4F (x = 0, 1) samples show the triclinic structure with P-1 space group. In the case of delithiated FeSO4F, changes in lattice parameters were observed due to the disappearance of the Li site while maintaining the triclinic structure. Temperature-dependent magnetic susceptibility curves show the antiferromagnetic (AFM) structure with the ordering temperature of 28 K for LiFeSO4F and 99 K for FeSO4F. Also, temperature-dependent Mössbauer spectra of LiFeSO4F below TN were fitted with two-sets of eight Lorentzian indicating the existence of two different types of Fe2+ ions due to the different hyperfine and electric quadrupole interactions. With the lithium delithiated, the Fe3+ iron ions in FeSO4F exhibit two-sets of six-line patterns due to the electron transfer in FeO4F2 site by lithium delithiation. These suggest that the different AFM ordering in LixFeSO4F can be originated from the Fe2+/Fe3+ magnetic ions with different charge distributions and spin contributions.
Journal of Applied Physics 03/2012; 111(7). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Fe<sub>3</sub>O<sub>4</sub> core/mesoporous SiO<sub>2</sub> shell (Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>) microspheres were prepared by a solvothermal reaction method. The crystal structure was determined to be cubic spinel with lattice constant a<sub>0</sub> of 8.395 Å for core Fe<sub>3</sub>O<sub>4</sub>. Based on transmission electron microscopy (TEM) measurements, the core of Fe<sub>3</sub>O <sub>4</sub> particle diameter is 300-500 nm and shell thickness of 50 nm. From the magnetic hysteresis curves measured under 10 kOe, magnetization of Fe<sub>3</sub>O<sub>4</sub> and Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub> microspheres is determined to be 77.0 and 17.0 emu/g , respectively, at room temperature. The M-T curve confirmed that the magnetic moment transition temperature was around 110 K in Fe<sub>3</sub>O<sub>4</sub> and 32 K in Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>. The Mössbauer spectra of the samples were analyzed with three six-line hyperfine patterns. It is noticeable that from the Mössbauer absorption area ratio between A(8a) and B(16d) sites, the area ratio of sextet increases from 40:60 for Fe<sub>3</sub>O<sub>4</sub> to 56:44 for Fe<sub>3</sub>O<sub>4</sub>@SiO<sub>2</sub>, respectively. The Fe valence state of A site was determined to be ferric, and B (B<sub>1</sub>, B<sub>2</sub>) site was ferric (B<sub>1</sub> site) and ferrous (B<sub>2</sub> site) from the isomer shift values.
IEEE Transactions on Magnetics 11/2011; · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report a novel methodology to prepare hierarchical structured Au–Co3O4 materials composed of Co3O4nanowires decorated with Au peripheral inclusions. The electrochemical activity of these materials was also demonstrated by fabrication into negative electrodes for rechargeable Li-batteries.
Journal of Materials Chemistry 09/2011; 21(37):14163-14166. · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The phase transition in sodium deintercalated Na2−xFePO4F (0 ≤ x ≤ 1) polycrystalline samples was studied by x-ray diffraction and Mo¨ssbauer spectroscopy. Sodium deintercalated samples were obtained by chemical deintercalation of pure Na2FePO4F. From the refined x-ray diffraction patterns, the crystalline structure of Na2FePO4F was determined to be orthorhombic with the space group Pbcn. The structure of fully sodium deintercalated NaFePO4F is identical to that of pure Na2FePO4F, differing only in the cell parameters. The changes in the unit cell parameters and atomic positions in Na2−xFePO4F (x = 0, 1) samples originated from a Na2 ion deficiency in the interlayer spaces. The room-temperature Mo¨ssbauer spectra of Na2−xFePO4F (x = 0, 1) were fitted with one set of the Fe2+ /3+ doublets. A decrease in the absorption area of Fe2 + in Na2FePO4F with increasing sodium deintercalation was observed in the Mo¨ssbauer spectra of Na2−xFePO4F, whereas the area of the Fe3+ doublet in NaFePO4F increased. The large value of ΔEQ for the Fe2+ doublet phase was due to the asymmetric charge distribution of FeO4F2 arising from different lattice and valence state contributions.
Journal of Applied Physics 04/2011; 109(7). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this report, functional one-dimensional (1-D) Pt-Co3O4 heterostruc-tures with enhanced electrochemical properties were synthesized via colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Colloidal polymerization of dipolar nanoparticles into hollow metal−semiconductor nanowires was achieved via a consecutive galvanic replacement reaction between Co0 and Pt2+ precursors, followed by a nanoscale Kirkendall oxidation reaction and a calcination treatment. X-ray diffraction (XRD), transmission electron microscopy (TEM), high-angle annular dark field scanning TEM (HAADF-STEM), and field-emission scanning electron microscopy (FESEM) revealed the structural and morphological evolution of the hollow cobalt oxide nanowires (D = 40 nm) with platinum nanoparticles (PtNPs; D 2 nm) entrapped within the growing oxide shell. Various calcination conditions were investigated via X-ray photoelectron spectroscopy (XPS) to obtain the optimal surface composition of the metallic Pt and semiconducting Co3O4 phases. Cyclic voltammetry of the 1-D Pt-Co3O4 heterostructures demonstrated a sevenfold enhancement in specific capacitance in comparison to the pristine Co3O4 nanowires. Preliminary results also showed that the calcined 1-D Pt-Co3O4 heterostructures catalytically hydrogenate methyl orange, and the rates of the hydrogenation were dependent on surface composition.Keywords: colloids; hybrid inorganic/organic materials; magnetic materials
[show abstract][hide abstract] ABSTRACT: The preparation of gold nanoparticle (AuNP) assemblies was conducted by the synthesis and dipolar assembly of ferromagnetic core-shell nanoparticles composed of AuNP cores and cobalt NP shells. Dissolution of metallic Co phases with mineral acids afforded self-assembled AuNP chains and bracelets.
Chemical Communications 11/2010; 47(3):890-2. · 6.38 Impact Factor
[show abstract][hide abstract] ABSTRACT: Polycrystalline samples of Li0.59FePO4 were prepared by oxidation reaction of pure LiFePO4, and temperature dependent valence states and magnetic properties were studied. X-ray diffraction patterns showed a biphasic olivine-type orthorhombic structure (space group: Pnma) where triphylite (LiFePO4) and heterosite (FePO4) coexisted. Also, we have observed the biphasic antiferromagnetic ordering of triphylite and heterosite with different antiferromagnetic to paramagnetic ordering transition temperatures. Mössbauer spectra of Li0.59FePO4 showed a two-phase asymmetrical eight line pattern due to the different electric quadrupole interactions in triphylite and heterosite. The iron ions of triphylite and heterosite are at Fe2+ (3d6) and the Fe3+ (3d5) valence state. Also the large value of ΔEQ for the triphylite phase was originated from the asymmetric charge distribution of FeO6. These results indicate that the charge distributions around the Fe nucleus in the triphylite are more asymmetric because of the contribution from the crystal field and the ion valence state.
Journal of Applied Physics 05/2010; 107(9). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The preparation of cobalt oxide nanowires with gold nanoparticle (AuNP) inclusions (Au-Co(3)O(4) nanowires) via colloidal polymerization of dipolar core-shell NPs is reported. Polystyrene-coated ferromagnetic NPs composed of a dipolar metallic cobalt shell and a gold NP core (PS-AuCoNPs) were synthesized by thermolysis of octacarbonyldicobalt [Co(2)(CO)(8)] in the presence of AuNP seeds and polymeric ligands. The colloidal polymerization process of these dipolar PS-AuCoNPs comprises dipolar nanoparticle assembly and solution oxidation of preorganized NPs to form interconnected cobalt oxide nanowires via the nanoscale Kirkendall effect, with AuNP inclusions in every repeating unit of the one-dimensional mesostructure. Calcination of the polymer-coated nanowires afforded polycrystalline Au-Co(3)O(4) nanowires that were determined to be electroactive. Nanocomposite materials were characterized by transmission electron microscopy, field-emission scanning electron microscopy, X-ray diffraction, vibrating sample magnetometry, and cyclic voltammetry. We demonstrate that the optical and electrochemical properties of Au-Co(3)O(4) nanowires are significantly enhanced in comparison with hollow Co(3)O(4) nanowires prepared via colloidal polymerization.
Journal of the American Chemical Society 02/2010; 132(10):3234-5. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: The synthesis of ferromagnetic cobaltnanoparticles with polymeric ligands with tunable particle size is reported on multi-gram scales. End-functional polystyrene ligands were prepared via atom transfer radical polymerization. Mechanistic studies on this system were conducted as well as direct comparisons of polymervs. small molecule ligand effects on nanoparticle formation.
Journal of Materials Chemistry 01/2010; 20(29). · 5.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The preparation of polystyrene-coated cobalt oxide nanowires is reported via the colloidal polymerization of polymer-coated ferromagnetic cobalt nanoparticles (PS-CoNPs). Using a combination of dipolar nanoparticle assembly and a solution oxidation of preorganized metallic colloids, interconnected nanoparticles of cobalt oxide spanning micrometers in length were prepared. The colloidal polymerization of PS-CoNPs into cobalt oxide (CoO and Co(3)O(4)) nanowires was achieved by bubbling O(2) into PS-CoNP dispersions in 1,2-dichlorobenzene at 175 degrees C. Calcination of thin films of PS-coated cobalt oxide nanowires afforded Co(3)O(4) metal oxide materials. Transmission electron microscopy (TEM) revealed the formation of interconnected nanoparticles of cobalt oxide with hollow inclusions, arising from a combination of dipolar assembly of PS-CoNPs and the nanoscale Kirkendall effect in the oxidation reaction. Using a wide range of spectroscopic and electrochemical characterization techniques, we demonstrate that cobalt oxide nanowires prepared via this novel methodology were electroactive with potential applications as nanostructured electrodes for energy storage.
[show abstract][hide abstract] ABSTRACT: Cobalt nanoparticles were synthesized by modified thermal decomposition and oxidized in the condition of solution. Crystal structure of fabricated cobalt nanoparticles is determined to be cubic of Fm-3m space group from X-ray diffraction (XRD) measurement. Microstructure of cobalt nanoparticles after the oxidation process show core-shell structure. The particle size and thickness of oxide shell can be controlled by oxidation temperature. The shell thickness of cobalt nanoparticles after the oxidation at 300degC is 4.7 nm, when compared to those oxidized at 250degC with the shell thickness of 2.4 nm due to the change in the oxidation temperature. The magnetic properties of cobalt particles have been measured with VSM. Our results show that cobalt metal (core)/cobalt oxide (shell) has two magnetic behaviors of ferromagnetic and antiferromagnetic properties in magnetic fluid.
IEEE Transactions on Magnetics 07/2009; · 1.42 Impact Factor